1. Technical Field
The present invention relates to a transmission method and a transmission device, and a reception method and a reception device, and a system using them. More particularly is related to such ones within communications systems which are using OFDM (Orthogonal Frequency Divisional Multiplexing).
2. Prior Art
An information transmission system generally transmits symbols, where each symbol for instance can be a sequence of ones and zeros in succession over a transmission channel, and there occupies a frequency band which of necessity must be wider/larger than the inverse of the time length of a symbol.
When the transmission speed is increased it finally will be impossible to guarantee that the transmission channel retains identical amplitude and phase characteristics over the whole frequency range which constitutes the pass band. These in this way developed distortions in the channel give rise to interference between symbols, which interference can be fought against by means of an equalizing device, a so called equalizer. Such systems, however, are rather complex.
One technology to handle/manage this problem includes that the signal which shall be transmitted is spread/distributed over a large number of carriers in a parallel way, individually modulated with/by low speed. Because the speed is low, the pass band width which is needed is smaller, and therefore it is more probable that amplitude and phase characteristics will be identical for all frequencies which constitute this band. This technology is known to the expert as “Orthogonal Frequency Divisional Multiplexing” or OFDM. Frequency spectra of the signals which modulate the carriers overlap in such a way that they fulfill the conditions for orthogonality, which makes elimination of interference between modulated sub-carriers possible and also makes it possible to achieve much larger spectral benefit.
The space between two adjacent sub-carriers corresponds to the inverse of the time length of a symbol.
The OFDM-modulation is usually incorporated with a Fourier-transform, so that it can be implemented by means of FTT (Fast Fourier Transform). The main steps to implement transmission of a message by means of OFDM-modulation is specified below.
First of all the binary data which constitute the message which shall be transmitted in data blocks are grouped. Each one of these blocks is transmitted independent of each other and constitutes, after base band modulation, an OFDM-signal. In each data block also the binary digits are grouped in subset. Each subset after that is subject to a “bijective mapping” over a discrete amount of points in the Fresnel-space, where each point represents a possible phase and amplitude. If, for instance, a message consisting of the following series of bits (00001110010001111000 . . . ) is considered, it will be possible to from that extract a block of 16 bits 0000111001000111, with which are associated, by mapping, the following amount of points in the complex plane:1+j, 1+j, −1−j, 1−j, −1+j, 1+j, −1+j, −1−j.This consequently gives an amount consisting of eight complex elements, which define a vector V.
An inverse discrete Fourier Transform with a matrix A then is allowed to be active/influence on the vectors V which have been obtained from the original message, which gives rise to an OFDM-signal consisting of a series of complex amplitudes.
Each transmitted/transferred symbol then is received, after having passed the transmission channel, by a demodulator, from which there is extracted a vector V which holds complex elements, by multiplying the amplitudes which describe the symbol by/with a direct discrete Fourier-transform matrix A′ so that A*A′=I, where I indicates the unit matrix.
The use of a decision criterion based on “Maximum likelihood” on the real part and on the imaginary part of each vector V′ makes regain of the original symbol sequence possible and further reconstruction of the to that associated binary elements.
The different symbols in each block are linked up due to the linear combination which is obtained by multiplying the elements in the transmitted vector V by the inverse discrete Fourier-transform matrix A. This linear combination guarantees a certain degree of hardiness and protects the symbols against interference between complex symbols within one and the same OFDM-symbol.
On the other hand, this protection/guard effect does not extend from one OFDM-symbol to another, that is, not from one block to another.
In order to prevent interference between blocks, it is known that a technology can be used which includes to arrange a time period of silence or non-transmission, also called guard (time) interval, between to successive symbols.
In prior art, however, the guard interval preceding current symbol is decided pragmatically, usually after an evaluation by an expert, of the time period which is necessary to attenuate the echo of the transmission of preceding OFDM-symbol.
Some variants including adjustment of the guard interval are also described below.
U.S. Pat. No. 6,115,354-A shows a method which adapts the “guard intervals for the OFDM symbols” to those differences in delay which exist in the network. The first guard interval for a frame, however, is adjusted to “worst case” (see column 2, line 9-column 3, line 9). According to this document, the flexibility of the guard interval results in that the OFDM-system can be optimized both from implementation and network planning perspective (see column 3, lines 36-40).
U.S. Pat. No. 6,175,550-B1 shows an OFDM-system in which a “guard time interval” is adjusted dynamically depending on the communication conditions in the environment (see column 3, lines 3-65), column 6, lines 24-32, and independent patent claims).
EP-1065855-A1 shows adjustment of “cyclic extensions” in an OFDM-system. The length of the cyclic extension is adjusted to the delays which are existing at/in the channel. (See abstract)
WO97/30531-A1 says that a “guard space” can be varied so that a minimal guard space is used (see patent claims).
EP-1061687-A1 shows automatic adjustment of “guard interval” depending on the quality of received signal.
EP-1014639-A2 shows an OFDM-transmitter/receiver for which an optimal selection of guard interval is decided.